1. Field of the Invention
This invention relates to mirrors for semiconductor laser systems and, more particularly, to vertical cavity surface emitting lasers implemented in semiconductor material systems containing at least one of indium and gallium and at least one of phosphorus and arsenic.
2. Related Art
For convenience the term "Vertical Cavity Surface Emitting Laser" is shortened to VCSEL.
The structure of a VCSEL includes two mirrors, e.g. distributed Bragg reflectors, which are located at opposite ends of the lasing zone. In some VCSELs the mirrors are part of the electrical system. The mirrors are required to maintain the lasing process by returning light back to the active region to stimulate more emission. To achieve satisfactory lasing performance VCSELs require good performance from the mirrors and the combined effect of the two mirrors is important. Reflectivities exceeding 99% are usually regarded as necessary. The reflectivity of the two mirrors may be different. The use of two high reflectivity mirrors, e.g. more than 99.5%, is, of course, desirable but other considerations sometimes make it appropriate to use a lower performance mirror in conjunction with a higher performance mirror.
At least one of the mirrors, identified as the "front mirror" in this specification, is required to transmit a suitable proportion of the radiation to act as the output of the VCSEL. For the front mirror 100% reflectivity is, therefore, not appropriate. It is convenient to call the other mirror the "rear mirror".
In order to provide the appropriate reflectivities the mirrors are usually implemented as several layers of suitable semiconductor material. More specifically, two different materials are utilised in the mirrors. The optical and physical theory suggests suitable refractive indices for the materials and the number of layers required to provide a given reflectivity. However the physical theory provides little help in identifying suitable materials for the two layers. More particularly, the physical theory does not help to define the chemical composition of the two layers.
In addition to providing appropriate refractive indices the following material properties are appropriate.
(A) The lattice constant of the mirror material must match the lattice constant of the lasing zone. Usually the lasing zone has a lattice constant substantially equal to that of indium phosphide and, therefore, is it desirable that the mirror match this constant. PA0 (B) The mirrors work by reflecting radiation from internal interfaces in such a manner that multiple reflections reinforce one another. Thus it is necessary for reflected radiation to pass through the layers of the mirror. In fact there is a double pass because incident radiation traverses the layers on the way in and reflected radiation repeats the traverse on the way out. Therefore, high reflectivities require that the layers must have good transmission properties. (The ideal would be 100% but this may not always be achieved in practice.) In addition, the output of the device traverses the front mirror and this is another reason why high transmission is needed. PA0 (C) VCSELs are technically and commercially significant because they are relatively cheap to manufacture. The radiation is transmitted normal to the substrate and the various epitaxial planes. This means that the lasers can be operated in the wafer before division. This is valuable for testing. It is also important for those applications which require an array of lasers. PA0 (D) It is also important that the materials from which the mirrors are made are stable and have an adequate life. In a VCSELs the mirrors define the limits of the lasing system and, in some VCSELs, the mirrors are part of the electrical system, e.g. they transmit the drive current. For this reason it is appropriate to form the mirrors out of semiconductor materials. PA0 (E) When the mirrors are part of the electrical system, the materials from which they are made should have adequate electrical conductivity. It is usually necessary to be able to provide both p-type and n-type conductivity. Thus it is desirable to select semiconductor materials which are able to be doped so as to provide either p-type or n-type conductivity as required.
It is difficult to select materials which offer adequate performance in respect of all of these requirements. This invention relates to semiconductor materials which are at least adequate in respect of all of them.
In Electronic Letters published 16th January 1997, Volume 3 (No. 2) at pages 140 and 141 Genty et al described high reflectivity distributed Bragg reflector structures for 1.5 .mu.m surface emitting lasers. The reflector structures are composed of alternate layers of Te-doped GaAsSb and un-doped AlAsSb. It is stated that the Te-doping gives the structure n-type conductivity. Without the doping the GaAsSb is highly absorbent. FIG. 2 of the publication shows that a maximum reflectivity exceeding 94% was measured on a mirror with only 11.5 period in the stack. FIG. 2 also shows that, even with n-type doped layers, there is low reflectivity at wavelengths below 1.4 .mu.m.
Anan et al in Electronic Letters 30, 2138 (1994), describe mirrors comprising AIPSb in one layer and GaPSb in the other layer.